This invention relates to gas turbine combustors; and, in particular, to improvements in gas turbine combustors for the further diminishment of air pollutants such as nitrogen oxides (NOx).
In an effort to reduce the amount of NOx in the exhaust gas of a gas turbine, inventors Wilkes and Hilt devised the dual stage, dual mode combustor which is shown in U.S. Pat. No. 4,292,801 issued Oct. 6, 1981 to a common assignee of the present invention. In this aforementioned patent, which is incorporated herein by reference, it was discovered that the amount of exhaust NOx could be greatly reduced, as compared with a conventional single stage, single fuel nozzle combustor, if there were two combustion chambers established such that under conditions of normal operating load, the upstream primary combustion chamber performed as a premix chamber whereas actual combustion occurred in the downstream second combustion chamber. Under this described operating condition, there would be no flame in the primary chamber resulting in a decrease in the formation of NOx. In this condition of operation, the secondary or center nozzle provides the flame source for the operation of the combustor. The specific configuration of the patented invention includes an annular array of primary nozzles each of which discharges into the primary combustion chamber and a central secondary nozzle which discharges into the second combustion chamber. These nozzles may all be described as diffusion nozzles in that each nozzle has an axial fuel delivery pipe and is surrounded at its discharge end by an air swirler which provides air for combustion to the fuel nozzle discharge. The present inventors have discovered that further reduction in the production of NOx can be achieved by altering the design of the central or secondary nozzle such that it may be described as a combined premix and diffusion nozzle. In operation, a relatively small amount of fuel is used to sustain a diffusion pilot whereas a premix section of the nozzle provides additional fuel for ignition of the main fuel supply from the upstream primary nozzles directed into the primary combustion chamber.